mutiphase+turbulence

I try to model a stratified multiphase flow (oil+water) in a 5m long pipe 144mm diameter with a velocity of 1m/s. At that velocity the flow should be very turbulent but when I modele it in fluent the flow reamin perfectly stratified... Do you have any suggestion to make on the models I should use with Fluent to observe the instabilities?

Do you have any experimental data indicate that in this case the flow is instable? If so you can add a impulse to the inlet boundary condition. By the way, for the oil phase, the flow may be laminar because of the viscosity of the oil.